The choice of method for calculating the ascent/descent (subscription)

When you import a gpx or kml file in expert mode, Trace de Trail will ask you the origin of this file : you must state whether the given data stems from measurements taken by GPS or not (route tracked or reworked from software or cartographic site). In the first instance, the altitudes used for the calculation of the ascent / descent are those of your file. In the second instance, Trace De Trail will use a Digital Elevation Model (DEM) to obtain the altitudes associated with your points. This service optimises the calculation methods to guarantee the greatest precision possible when calculating your ascent / descent.

In create or modify mode of the track, your track will be posted on map in one of two colours: green or blue. Green tracks use a GPS to gather the altitude points while the DEM will be used for the blue.

Also, if you import a gpx file from your GPS, your track will be totally green. On the other hand, if you modify your track by moving or adding summits, these latter will appear in blue. The following image shows an example of a track from a gpx file (points in green), and partly modified manually (points in blue). The proportion of the distance with points measured by GPS is 89%.

The interest of this distinction is to be able to choose the most apt method for calculating the ascent / descent during the stage of creating a profile. If less than 90% of the track distance is compiled with measurements taken by GPS, we advise you to use the ASTER GDEM . However, if the ratio is superior to 90%, you choose between an ascent/descent threshold of 3M (if you have taken the readings with a GPS with a barometric altimeter or a threshold of 10m (GPS without barometric altimeter). If you do not know which type of GPS was used, we advise you to use a threshold of 10m by default.

This threshold corresponds to the minimal difference of ascent/descent between two successive points so that the latter is recorded. The use of a threshold allows the partial deletion of the variations in ascent/descent connected with the inaccuracies of the measurements.

The interest in this is to know whether you can have your route certified by the ITRA. Actually, so that a route can be certified, it must have 90% of the track registered by GPS on the ground. Should the opposite occur your route can only be estimated. To know more...